The present invention relates to systems and methods for generating hydrogen, and more particularly to systems and methods for generating hydrogen gas at pressures high enough to fill gas storage cylinders.
Hydrogen gas must be generated at high pressures to fill hydrogen storage cylinders for stationary and transportation applications, including on board a vehicle and at refueling stations. To produce hydrogen for use or storage at high pressure, water electrolysis may be performed at the required high pressure, generating both hydrogen and oxygen at high pressure. Alternatively, differential-pressure electrolysis may be employed to generate hydrogen at high pressure and oxygen at substantially atmospheric pressure. To date, high pressure water electrolyzers have been fabricated that either generate both hydrogen and oxygen at 3000 psia, where psia is the pressure in pounds per square inch, absolute, or generate hydrogen at 2500 psia and oxygen at atmospheric pressure. For example, Giner Electrochemical Systems, LLC has fabricated a water electrolyzer that operates at a differential pressure (H2 greater than O2) of 2500 psia using plastic materials as frames and proton-exchange membranes (PEMs) as solid-polymer electrolytes. A low-pressure pump provides liquid water at near-ambient pressure to the anode side of the electrolyzer. When DC current is applied, the water is decomposed at the anode to oxygen, protons and electrons. The oxygen is separated from the excess circulating water, which acts as a reactant and coolant, with a low-pressure gas/water separator. All functions on the anode side are conducted at near-ambient pressure. The protons, along with some water, are electrochemically transported across the membrane to the cathode, where they react with the externally transported electrons to produce hydrogen at the required higher operating pressure. The hydrogen is separated from the transported water in a high-pressure gas/water separator.
Electrolyzers operating totally or partially at high pressure may be expensive, involve complex construction, and present safety hazards. Therefore, a need exists in the art for simple, safe, and inexpensive systems and methods for generating hydrogen gas at high pressures.
The systems of the present invention can generate hydrogen gas at pressures high enough to fill a gas storage cylinder for stationary and transportation applications, including on board a vehicle and at refueling stations. The electrochemical process for generating hydrogen at pressures that may be greater than 3000 psia features feeding the hydrogen output of a water electrolyzer or related electrochemical hydrogen gas generating device operated at atmospheric or moderate pressure to an electrochemical hydrogen compressor operating in a high-differential-pressure mode. xe2x80x9cAtmospheric or moderate pressure,xe2x80x9d as used herein, means from about 0 psia to about 3000 psia. The electrochemical hydrogen compressor has an anode operating at the same pressure as the cathode of the electrochemical hydrogen generator and a cathode operating at the higher pressure required to fill the gas storage cylinder. The compressor, which may be operated at a 3000 psia or greater pressure differential, elevates hydrogen produced by the electrochemical hydrogen generator to the desired high pressure, for example, 6000 psia.
The electrochemical hydrogen generator and compressor of the invention are stacks comprising one or more cells connected electrically in series or in parallel. In some preferred embodiments, each cell contains a membrane and electrode assembly (MEA) comprising an anode and a cathode in intimate contact with and separated by an ionic conductive membrane such as a proton-exchange membrane (PEM) or solid alkaline membrane. When power is applied to each cell in the electrochemical hydrogen generator stack, protons and electrons are generated at the anode. The protons are electrochemically transported across the membrane to the cathode, where they combine with the externally transported electrons to form hydrogen gas. This hydrogen gas is fed to the hydrogen compressor, where it is oxidized at the anode of each cell to form protons and electrons. The protons are transported across the membrane to the cathode, where they are reduced by the externally transported electrons to form hydrogen at the desired higher pressure.
The anticipated benefits of the invention include safety of operation and relative simplicity of constructing a differential-pressure hydrogen compressor cell compared to an electrolyzer with the same pressure difference, which translates into cost savings. The two-cell system of the invention is safer to operate than a high-pressure electrolyzer. Membrane failure in the compressor cell presents little hazard as long as there is a pressure shut off valve, and membrane failure in the low-pressure electrolyzer is less dangerous than it would be in a high-pressure electrolyzer. Thus, the two-cell system allows for the use of thinner membranes, resulting in lower voltage. This compensates for the somewhat higher overall voltage and power inefficiency anticipated when using two cells instead of one. In addition, less risk of explosion exists in recirculating water accumulated at the anode or cathode side of the compressor to the low-pressure electrolyzer of the two-cell system than in feeding a pressurized reactor with cathode water, even if the water contains some hydrogen.
A water electrolyzer alone could be used to generate hydrogen at high pressure; however, above 2500 psia differential pressure, difficulties arise in supporting the MEA as mechanical properties of the membrane, metallic support structures and compression pads rapidly deteriorate. A high-pressure PEM electrolyzer is also more expensive than an integrated low-pressure electrolyzer and electrochemical hydrogen compressor. Low-cost materials that may be used in the compressor, but not in the high-pressure electrolyzer, include carbon-supported electrode structures, stainless steels, inconels, hastelloys, low-cost hydrocarbon PEMs, and anion exchange (hydroxide transport) membranes. In the compressor, carbon, graphite, hastelloys, stainless, and inconels may replace the costly valve metals (Ti, Zr, Nb) used in electrolyzers. In addition, small noble metal loadings are required due to the high reversibility of the hydrogen electrode in the absence of carbon monoxide and other inhibiting gas traces.
Further, the sizable power efficiency losses associated with operating a pressurized hydrogen cathode are an order of magnitude smaller in hydrogen compressor cells than in a high-pressure (or high-pressure-differential) water electrolyzer, because the cell voltage is an order of magnitude lower. Operating the electrolyzer at near atmospheric pressure also allows for the use of lower current densities at the electrolyzer stack without a substantial decrease in faradaic efficiency, which is close to 100%. Decreased current density may be achieved by distributing approximately the same amount of electro-catalyst over a larger membrane surface, resulting in higher voltage efficiency of the electrolyzer. These advantages more than compensate for the additional voltage required by the hydrogen compressor cell (which may contribute 5 to 10% to the overall system voltage compared to single cell voltage) and the existence of two cells versus one.
These and other benefits and features of the present invention will be more fully understood from the following detailed description, which should be read in light of the accompanying drawings.